Virus-cell interaction during various developmental stages of mouse mammary tumors is being investigated. The mammary gland plaques and early mammary tumors from GR and DD strains of mice were examined by electron microscopy to study in vivo transformation of mammary cells. In plaques the mammary epithelial cells form small lobules with a central ductule in which a large number of extracellular virus particles were observed. The nodule mammary cells do not show significant changes as compared to plaque cells, and there is an increase of intracytoplasmic type A particles but very few extracellular virus particles were observed. Electron microscopy of human mammary and renal carcinomas was continued to identify virus or virus-related morphological entities to correlate with RNA dependent DNA polymerase activity in cancer cells. Preliminary study has failed to reveal any virus-like morphological entities. Electron microscopic search of virus particles in 40 dog mammary tumors was completed. However, virus-like structures observed in three tumors could not be confirmed as virus particles.